Field of the Invention
The present invention relates to wireless communication and, more particularly, to an method of operating a user equipment which secures the continuity of a device-to-device (D2D) operation at the time of reselecting a cell, and a user equipment using the method.
Related Art
In an International Telecommunication Union Radio communication sector (ITU-R), a standardization of International Mobile Telecommunication (IMT)-Advanced being a next mobile communication system after a third generation has been performed. The IMT-Advanced is aimed at supporting an Internet Protocol (IP) based multi-media service with a data transmission rate of 1 Gbps in a stop and low speed moving state and a data transmission rate of 1 Gbps in a high speed moving state.
A 3rd Generation Partnership Project (3GPP) is preparing LTE-Advanced (LTE-A) being an improved one of Long Term Evolution (LTE) based on an OFDMA (Orthogonal Frequency Division Multiple Access)/SC-FDMA (Single Carrier-Frequency Division Multiple Access) transmission scheme as a system standard satisfying requirements of IMT-Advanced. The LTE-A is one important candidate for IMT-Advanced.
In recent years, there is growing interest in a Device-to-Device (D2D) technology performing direct communication between devices. In particular, the D2D is attracting attention as a communication technology for a public safety network. A commercial communication network has been rapidly changed to the LTE but a current public safety network is based on a 2G technology in a collision problem and a cost side with an existing communication standard. Request for the technology clearance and an improved service induces an effort to improve the public safety network.
The public safety network has high service requirements (reliability and security) as compared with a commercial communication network. In particular, when coverage of cellular communication is insufficient or is not used, there is a need for direct signal transmission/reception between devices, that is, a D2D operation.
The D2D operation may be signal transmission/reception between adjacent devices to have various advantages. For example, a D2D terminal may perform data communication with a high transmission rate and low delay. Further, the D2D operation may distribute traffic converged in a base station. If the D2D terminal serves as a relay, the D2D terminal may serve to extend coverage of a base station.
In the device-to-device (D2D) operation, there may be a case that reliability is important. For example, on emergency or in a disaster situation, when the D2D operation is used, the breakdown of the continuity of the D2D operation is not desirable.
When the channel state with current serving cell is not good, the UE may perform a RRC (radio resource control) connection reestablishment procedure. At the RRC connection reestablishment procedure, the cell reselection may be performed. However, the cell selected by the UE through the cell reselection process may not support the D2D operation or may not provide the setting about the D2D operation. If a cell not providing the setting about the D2D operation is selected through the cell reselection process while the UE performs the D2D operation in the existing cell, the operation of the UE may be a problem.